Error with the auto forward mails

Hello guys,

I have a little problem and I don't know what else to try. A couple of my clients want to forward automatically all the mails in a mailbox to accounts in outlook (hotmail and outlook365).

If I change the config the forward to gmail it works properly. If I send mails to hotmail manually it works too.

What should I check? I already know outlook has probably one of the most strict protocols, but... don't know...

Any tips?

Regards,
Neo.